THE THORAX
The Thorax or chest is an osseo-cartilaginous cage, intended to contain and protect the principal organs of respiration and circulation. It is the largest of the three cavities connected with the spine, and is formed by the sternum and costal cartilages in front, the twelve ribs on each side, and the bodies of the dorsal vertebrae behind.
THE STENUM
The Sternum (figs. 56, 57) is a flat narrow bone, situated in the median line of the front of the chest, and consisting in the adult of three portions. Its form resembles an ancient sword: the upper piece representing the handle, is termed the manubrium, the middle and largest piece which represents the chief part of the blade, is termed the gladiolus, and the inferior piece like the point of the sword, is termed the ensiform or xiphoid appendix. In its natural position, its direction is oblique, its anterior surface looking upwards and forwards, its posterior downwards and backwards. It is flattened in front, concave behind, broad above, becoming narrowed at the point where the first and second pieces are connected, after which it again widens a little, and is pointed at its extremity.
The Stenum
The First Piece of the sternum or Manubrium, is of a somewhat triangular form, broad and thick above, narrowed below at its junction with the middle piece. Its anterior surface convex from side to side, concave from above downwards, is smooth and affords attachment on each side to the Pectoralis major and sternal origin of the Sterno-cleido-mastoid muscle. In well marked bones, ridges limiting the attachment of these muscles are very distinct. Its posterior surface, concave and smooth, affords attachment on each side to the Sterno-hyoid and Sterno-thyroid muscles. The superior border, the thickest, presents at its centre the interclavi-cular notch, and on each side an oval articular surface, directed upwards backwards snd outwards, for articulation with the sternal end of the clavicle. The inferior border presents an oval rough surface, covered in the recent state with a thin layer of cartilage, for articulation with the second portion of the bone. The lateral borders are marked superiorly by an articular depression for the first costal cartilage, and below by a half facet, whict, with a similar facet on the upper angle of the middle portion of the bone, forms a notch for the reception of the costal cartilage of the second rib. These articular surfaces are separated by a curved edge which slopes from above downwards and inwards.
The Second Piece of the sternum, or gladiolus, considerably longer, narrower, and thinner than the superior, is broader below than above. Its anterior surface is nearly flat, directed upwards and forwards, and marked by three transverse lines which cross the bone opposite the third, fourth, and fifth articular depressions. These lines indicate the point of union of the four separate pieces of which this part of the bone consists at an early period of life. At the junction of the third and fourth pieces, is occasionally seen an orifice, the sternal foramen; it varies in size and form in different individuals, and pierces the bone from before backwards. This surface affords attachment on each side to the sternal origin of the Pectoralis major. The posterior surface, slightly concave, is also marked by three transverse lines; but they are less distinct than those on the anterior surface; this surface affords attachment below, on each side, to the Triangularis sterni muscle, and occasionally presents the posterior opening of the sternal foramen. The superior border is marked by an oval surface for articulation with the manubrium. The inferior border is narrow and articulates with the ensiform appendix. Each lateral border presents five articular depressions; the first, at each superior angle, is a half facet for the lower half of the cartilage of the second rib, the three succeeding depressions receive the cartilages of the third, fourth, and fifth ribs, whilst each inferior angle presents a half facet for the upper half of the cartilage of the seventh rib. These depressions are separated by a series of curved inter-articular notches, which-diminish in length from above downwards.
The Third Piece of the sternum, the ensiform or xiphoid appendix, is the smallest of the three; it is thin and elongated in form, cartilaginous in structure in youth, but more or less ossified at its upper part in the adult. Its anterior surface affords attachment to the costo-xiphoid ligaments. Its posterior surface, to some of the fibres of the Diaphragm and Triangularis sterni muscles. Its lateral borders, to the aponeurosis of the abdominal muscles. Above, it is continuous with the lower end of the gladiolus; below, by its pointed extremity, it gives attachment to the linea alba, and at each superior angle presents a half facet for the lower half of the cartilage of the seventh rib. This portion of the sternum is very various in appearance, being sometimes pointed, broad and thin, sometimes bifid, or perforated by a round hole, occasionally curved, or deflected considerably to one or the other side.
Structure. This bone is composed of a considerable amount of loose spongy tissue within, covered externally with a very thin layer of compact tissue.
Structure of Stenum
Development. The sternum, including the ensiform appendix, is developed by six centres. One for the first piece or manubrium, four for the second piece or gladiolus, and one for the ensiform appendix. The sternum is entirely cartilaginous up to the middle of foetal life, and when ossification takes place, the ossific granules are deposited in the middle of the intervals between the articular cartilages, in the following order. In the first piece, between the fifth and sixth months; in the second and third, between the sixth and seventh; in the fourth piece, at the ninth month; in the fifth, within the first year, or between the first and second years after birth: and in the ensiform appen dix, between the second and the seventeenth or eighteenth years, by a single centre which makes its appearance at the upper part, and proceeds gradually downwards. To these may be added the occasional existence, as described by Breschet, of two small epi-sternal centres, which make their appearance one on each side of the interclavicular notch. It occasionally happens that some of these divisions are formed from more than one centre, the number and position of which vary. Thus the first piece may have two, three, or even six centres; the second piece has seldom more than one; the third, fourth, and fifth pieces, are often formed from two centres placed laterally, the irregular union of which will serve to explain the occasional occurrence of the sternal foramen, or of the vertical fissure which occasionally intersects this part of the bone. Union of these various parts commences from below, and proceeds upwards, taking place in the following order. The fifth piece is joined to the fourth soon after puberty; the fourth to the third.between the twentieth and twenty-fifth years; the third to the second, between the thirty-fifth and fortieth years; the second is rarely joined to the first except in very advanced age.
Development of Stenum
Articulations. With the clavicles, and seven costal cartilages on each side.
Attachment of Muscles. The Pectoralis major, Sterno-cleido-mastoid, Sterno-hyoid, Sterno-thyroid, Triangularis sterni, aponeurosis of the Obliquua externus, Obliquus internus, and Transversalis muscles, Rectus and Diaphragm.
THE RIBS
The Ribs are elastic arches of bone, which form the chief part of the thoracic walls. They are twelve in number on each side; but this number may be increased by the development of a cervical or lumbar rib, or maybe diminished to eleven. The first seven are connected behind with the spine, and in front with the sternum, through the Intervention of the costal cartilages, they are called vertebra-sternal, or true ribs. The remaining five are false ribs; of these the first three, being connected behind with the spine, ana in front with the costal cartilages, are called the vertebra-costal ribs; the last two are connected with the vertebrae only, being free at their anterior extremities, they are termed vertebral or floating ribs. The ribs vary in their direction, the upper ones being placed nearly at right angles with the spine; the lower ones are placed obliquely, so that the anterior extremity is lower than the posterior. The extent of obliquity reaches its maximum at the ninth rib, gradually decreasing from that point towards the twelfth. The ribs are situated one beneath the other in such a manner that spaces are left between them; these are called intercostal spaces. Their length corresponds to the length of the ribs, their breadth is more considerable in front than behind, and between the upper than between the lower ribs. The ribs increase in length from the first to the eighth, when they again diminish to the twelfth. In breadth they decrease from above downwards; in each rib the greatest breadth is at the sternal extremity.
The Rib
Common characters of the Rib . Take a rib from the middle of the series in order to study its common characters. Each rib presents two extremities, a posterior or vertebral, an anterior or sternal, and an intervening portion, the body or shaft. The posterior or vertebral extremity, presents for examination a head, neck, and tuberosity.
The head is marked by a kidney-shaped articular surface, divided by a horizontal ridge into two facets for articulation with the costal cavity formed by the junction of the bodies of two contiguous dorsal vertebrae; the upper facet is small, the inferior one of large size; the ridge separating them, serves for the attachment of the inter-articular ligament.
The neck is that flattened portion of the rib which extends outwards from the head; it is about an inch long, and rests upon the transverse process of the inferior of the two vertebras with which the head articulates. Its anterior surface is flat and smooth, its posterior rough, for the attachment of the middle costo-transverse ligament. Of its two borders, the superior presents a rough crest for the attachment of the anterior costo-transverse ligament; its inferior border is rounded. On the outer surface of the neck, just where it joins the shaft, is an eminence, the tuberosity; it consists of two portions, an articular and a non-articular. The articular portion, the most internal and inferior of the two, presents a small oval surface directed downwards, backwards and inwards, for articulation with the extremity of the transverse process of the vertebra below it. The non-articulat portion is a rough elevation, which affords attachment to the posterior costo-trans-verse ligament.
Vertebral Extermity of a Rib
The shaft presents two surfaces, an external and an internal; and two borders, a superior and an inferior. The external surface is convex, and marked for the attachment of muscles. At its posterior part, a little in front of the tuberosity, is seen a prominent line, directed obliquely from above, downwards and outwards; this gives attachment to a tendon of the Sacro-lumbalis muscle, and is called the angle. At this point, the rib is bent upon itself in two directions. If the rib is laid upon its lower border, it will be seen that the anterior portion of the shaft, as far as the angle, rests upon this surface, while the vertebral end of the bone, beyond the angle, is bent inwards and at the same time tilted upwards. The distance between the angle and the tuberosity increases gradually from the second to the tenth rib. This portion of bone is rounded, rough, and irregular, and serves for the attachment of the Longissimus dorsi. The portion of bone between the angle and sternal extremity is also slightly twisted upon its own axis, the external surface looking downwards behind the angle, a little upwards in front of it. This surface presents, towards its sternal extremity, an oblique line, the anterior angle. The internal surface is concave, smooth, and presents the orifices of two or three nutrient foramina, the course of which is directly backwards towards the vertebral extremity. This surface looks a little upwards, behind the angle; a little downwards, in front of it. The superior border, thick and rounded, is marked by an external and an internal lip, more distinct behind than in front; they serve for the attachment of the External and Internal intercostal muscles. The inferior border, thin and sharp, has attached the External intercostal muscle. This border is marked on its inner side by a deep groove which commences at the tuberosity and gradually becomes lost at the junction of the anterior with the middle third of the bone. At the back part of the bone, this groove belongs to the inferior border; but just in front of the angle, where it is deepest and broadest, it corresponds to the internal surface; it lodges the intercostal vessels and nerve. Its superior edge is rounded and continued back as far as the vertebral extremity; it serves for the attachment of the Internal intercostal muscle. Its inferior edge corresponds to the lower margin of the rib, and gives attachment to the External intercostal. The anterior or sternal extremity, is flattened, and presents a porous oval concave depression, into which the costal cartilage is received.
PECULIAR RIBS
The peculiar ribs which require especial consideration, are five in number, viz., the first, second, tenth, eleventh and twelfth.
Thejirst rib is one of the shortest and the most curved of all the ribs; it is broad, flat, and placed horizontally at the upper part of the thorax, its surfaces looking upwards and downwards; and its borders, inwards and outwards. The head is of small size, rounded, and presents only a single articular facet for articulation with the body of the first dorsal vertebra. The neck is narrow and rounded. The tuberosity, thick and prominent, rests on the outer border. There is no angle, and it is not twisted on its axis. The upper surface of the shaft is marked towards its anterior part by two shallow depressions, separated from one another by a ridge, which becomes more prominent towards the internal border, where it terminates in a tubercle; this tubercle and ridge serve for the attachment of the Scalenus anticus muscle, the groove in front of it transmitting the subclavian vein; that behind it, the subclavian artery. The inferior surface is smooth, and destitute of the groove observed on the other ribs. The outer border is convex, thick, and rounded. The inner, concave, thin, and sharp, and marked about its centre by the tubercle before mentioned. The anterior extremity is larger and thicker than any of the other ribs.
Peculiar Ribs
The second rib is much longer than the first, but bears a very considerable resemblance to it in the direction of its curvature. The non-articular portion of the tuberosity is occasionally only slightly marked. The angle is slight, and situated close to the tuberosity, and the rib is not twisted, so that both ends touch any plane surface upon which it may be laid. The shaft is not horizontal, like that of the first rib; its external surface, which is convex, looking upwards and a little outwards; it presents near the middle a rough eminence for the attachment of part of the first, and the second serration of the serratus magnus. The inner surface smooth and concave, is directed downwards and a little inwards; it presents a short groove towards its posterior part.
The tenth rib has only a single articular surface on its head.
The eleventh and twelfth ribs have each a single articular surface on the head, which is of rather large size; they have no neck or tuberosity, and are pointed at the extremity. The eleventh has a slight angle and a shallow groove on the lower border. The twelfth has neither, and is much shorter than the eleventh.
Structure. The ribs consist of a quantity of cancellous tissue, enclosed in a thin compact layer.
Development. Each rib, with the exception of the last two, is developed by three centres, one for the shaft, one for the head, and one for the tubercle. The last two have only two centres, that for the tubercle being wanting. Ossification commences in the body of the ribs at a very early period, before its appearance in the vertebrae. The epiphysis of the head, which is of a slightly angular shape, and that for the tubercle, of a lenticular form, make their appearance between the sixteenth and twentieth years, and do not become united to the rest of the bone until about the twenty-fifth year.
Attachment of Muscles. The Intercostals, Scalenus anticus, Scalenus medius, Scalenus posticus, Pectoralis minor, Serratus magnus, Obliquus externus, Trans-versalis, Quadratus lumborum, Diaphragm, Latissimus dorsi, Serratus posticus superior, Serratus posticus inferior, Sacro-lumbalis, Musculus accessorius ad sacro-luinbalem, Longissimus dorsi, Cervicalis ascendens, Levatores costarum.
THE COSTAL CAETILAGES
The Costal Cartilages are white elastic structures, which serve to prolong the ribs forward to the front of the chest, and contribute very materially to the elasticity of this cavity. The seven first are connected with the sternum, the three next with the lower border of the cartilage of the preceding rib. The cartilages of the two last ribs, which have pointed extremities, float freely in the parietes of the abdomen. Like the ribs, the costal cartilages vary in their length, breadth, and direction. They increase in length from the first to the seventh, and gradually diminish to the last. They diminish in breadth, as well as the intervals between them, from the first to the last. They are broad at their attachment to the ribs, and taper towards their sternal extremities, excepting the two first, which are of the same breadth throughout, and the sixth, seventh, and eighth, which are enlarged where their margins are in contact. In direction they also vary; the first descends a little, the second is horizontal, the third ascends slightly, whilst all the rest follow the course of the ribs for a short extent, and then ascend to the sternum or preceding cartilage. Each costal cartilage presents two surfaces, two borders, and two extremities. The anterior surface is convex, and looks forwards and upwards; that of the first gives attachment to the costo-clavicular ligament; that of the first, second, third, fourth, fifth, and sixth at their sternal ends to the Pectoralis major. The others are covered, and give partial attachment to some of the great flat muscles of the abdomen. The posterior surface is concave, and directed backwards and downwards, the six or seven inferior ones affording attachment to the Transversalls and Diaphragm muscles. Of the two borders, the superior is concave, the inferior, convex; they afford attachment to the Intercostal muscles, the upper border of the sixth giving attachment to the Pectoralis major muscle. The contiguous borders also of the sixth, seventh, and eighth, and sometimes the ninth and tenth costal cartilages present smooth oblong surfaces at the points where they articulate. Of the two extremities, the outer one is continuous with the osseous tissue of the rib to which it belongs. The inner extremity of the first is continuous with the sternum; the six next have rounded extremities, which are received into shallow concavities on the lateral margins of the sternum. The inner extremities of the eighth, ninth and tenth costal cartilages are pointed, and lie in contact with the cartilage above. Those of the eleventh and twelfth are free and pointed.
In the male, the first costal cartilage becomes more or less ossified in the adult, and is often connected to the sternum by bone. Ossification of the remaining cartilages also occurs to a variable extent after the middle of life, those of the true ribs being first ossified. In the female, the process of ossification does not take place until old age. The costal cartilages are most elastic hi youth, those of the false ribs being more bo than the true. In old age they become of a deep yellow colour.
Attachment of Muscles. The Subclavius, Sterno-thyroid, Pectoralis major, Internal oblique, Transversalis, Bectus, Diaphragm, Triangularis sterni, Internal and External intercostals.
